Technical Field
This disclosure relates to switch mode power supplies and to spread spectrum technology.
Description of Related Art
Switch mode power supplies can have noise in their outputs due to switching that takes place in the power supply.
One approach for reducing the impact of this noise is to vary the frequency of the switching during operation of the switch mode power supply using spread spectrum technology.
Unfortunately, using spread spectrum technology can produce less than ideal results. The approach can reduce a birdie at the switching frequency and harmonics thereof, but can also generate additional noise on the output, impairing the performance as seen in the time domain.
Frequency hopping can be used to change the switching frequency. This approach changes the frequency at random, hopping from frequency to frequency. However, a new compensation node voltage in the switch mode power supply may need to be found for each frequency hop, since inductor ripple current may vary with frequency. A current mode feedback loop in the switch mode power supply may therefore need to settle to a new peak current after each frequency hop, impairing the performance of the power supply.
Frequency ramping can also be used to change the switching frequency. This approach modulates the clock in the switch mode power supply with a triangular wave to spread out the noise, but to keep the generated noise closer to the switching frequency, instead of over all frequencies. An example of this is described in U.S. Pat. No. 7,362,191. But the triangular wave can still generate multiple smaller birdies at the cost of somewhat smaller excursions on the output voltage in the time domain.